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    Issue Date: May 2023

    Live Microbes Intake Linked with Better Health

    • Previous research has focused on probiotic supplements or fermented foods, with little work on total dietary microorganisms and human health.
    • Researchers classified foods as being low, medium, or high in live microorganisms and estimated the overall amounts of microbes in people’s diets.
    • In a U.S. sample, people who consumed more live microbes had better markers of cardiometabolic health; more research is needed to determine if the relationship is causal.

    If you’ve given much thought to the healthfulness of your diet, you may have considered how many grams of protein or fiber you consume, or how many servings of fruits and vegetables you eat each day. But have you ever stopped to wonder how many living microbes are on your plate?

    It’s a question worth pondering, given that we live in a world teeming with microbes, trillions of which live on our bodies and within our gastrointestinal tracts—making up our microbiomes. Over the last several decades, more and more research has revealed the complex interactions between the microbiome and human health. And while there have been many studies on the effects of probiotic supplements and a smaller number on fermented foods, there has so far been little attention paid to the amounts of living microbes in our overall diets.

    Recently, an international group of scientists set out to change this, testing an overarching hypothesis that a dietary pattern that includes more live microbes is associated with better health [1]. Led by researchers at the University of California, Davis, University College Cork in Ireland, and the International Scientific Association for Probiotics and Prebiotics, the group worked to first develop a system of classifying foods as containing low, medium, and high numbers of live microorganisms, including bacteria, yeasts, and molds [2]. From there, they were able to estimate people’s total microbial intake from dietary records and look for associations with health outcomes, said Dan Tancredi, a statistician at UC Davis who was part of the team [3].

    “Despite the obvious public health benefits that more hygienic foods and environments provide, there may also be unforeseen negative health consequences as a result of these reductions in microbial exposures,” the researchers wrote in an opinion paper published in The Journal of Nutrition in 2020 [1], suggesting that our more sterile diets may have contributed to a shift in our gut microbiota that has coincided with the rise in chronic immune, metabolic, and other “lifestyle” diseases prevalent in the industrialized world [4].

    In 2022, the group published their system for classifying foods based on live microbe levels, along with an analysis of dietary records from 74,466 children and adults in the United States who participated in the National Health and Nutrition Examination Survey (NHANES) between 2001 and 2018 [2]. NHANES is a nationally representative survey of Americans that includes an in-person interview to capture participants’ dietary intakes, as well as a range of health assessments collected at mobile clinics [5].

    In the food classification scheme, the high-microbe food category included fermented foods, mostly dairy products such as yogurt, buttermilk, kefir, sour cream, and most cheeses. Medium-microbe foods included fresh vegetables, fruits, and fruit juices, which can acquire microorganisms during cultivation, harvest, and other steps in the supply chain. Most food items were classified as being low in live microbes, including foods that are generally cooked or heat-treated before consumption, such as milk, meat, poultry, seafood, sauces, gravies, and pasta.

    The researchers found that vegetables, fruits, and dairy products contributed most of the living microorganisms found in the diets of NHANES participants. On average, only about 5% of adult participants’ caloric consumption came from foods with medium to high levels of live microbes.

    In the group’s most recent study [3], published online in the Journal of Nutrition in February, they looked at associations between live microbe intake and several health parameters, again using data from NHANES collected between 2001 and 2018. Overall, they found that people who consumed more foods containing live microbes had lower systolic blood pressure, waist circumference, body mass index, and blood levels of glucose, insulin, triglycerides, and C-reactive protein, a marker of inflammation. They also had greater high-density lipoprotein, which is associated with lower risk of heart disease and stroke. The researchers reported similar positive associations for fermented food intake. Their analyses included adjustment for age, sex, ethnicity, poverty incomes ratios, physical activity, smoking status, alcohol intake, and body mass index.

    In other words, people who regularly ate more fermented foods or other foods containing live microorganisms appeared to be healthier. The study did not assess intake of probiotic supplements, because relatively few NHANES participants consumed them, and the researchers wanted to focus on the dietary contribution of live microorganisms, Tancredi said.

    The researchers estimated that people who ate 400 grams more of live microbe-containing foods (such as one serving each of yogurt, fruit, and vegetables) and 400 grams less of highly processed or heat-treated foods would be expected to have 1 to 4 mm Hg lower systolic blood pressure and waistlines that were two-thirds to two inches smaller.

    But this type of observational research has some inherent limitations, Tancredi emphasized. “We can’t unambiguously attribute the [live microbe] intakes with better outcomes in terms of cause and effect,” he said. People who eat more of foods like yogurt, fruit, and vegetables, might lead healthier lives in ways that weren’t accounted for in the study, such as getting more sleep or having less stress, or have other advantages, such as greater access to fresh foods. Plus, the same foods that contain live microbes may improve health through other mechanisms, such as nutrients, fiber, or phytochemicals.

    Much more research is needed, ideally randomized controlled trials that would include interventions to increase intake of live microbes and measure subsequent health effects, Tancredi said. “There’s a lot we don’t know and need to find out,” he added.

    That said, there’s plenty of evidence to support health benefits of eating fresh fruits and vegetables. And several recent randomized controlled trials have demonstrated that including yogurt or other fermented foods in your daily diet can reduce inflammation [6], improve gut microbiota diversity [7], and even reduce stress [8]. So, while we watch as more microbiome science emerges, it may be worthwhile to think of our meals as more than foods, nutrients, or fuel—but also microbial parties on our plates.


    1. Marco ML, Hill C, Hutkins R, Slavin J, Tancredi DJ, Merenstein D, Sanders ME. Should there be a recommended daily intake of microbes? Journal of Nutrition. 2020 Dec 150 (12):3061–7.
    2. Marco ML, Hutkins R, Hill C, Fulgoni VL, Cifelli CJ, Gahche J, Slavin JL, Merenstein D, Tancredi DJ, Sanders ME. A classification system for defining and estimating dietary intake of live microbes in US adults and children. Journal of Nutrition. 2022 Jul 152(7):1729–36.
    3. Hill C, Tancredi DJ, Cifelli CJ, Slavin JL, Gahche J, Marco ML, Hutkins R, Fulgoni VL, Merenstein D, Sanders ME. Positive health outcomes associated with live microbe intake from foods, including fermented foods, assessed using the NHANES database. Journal of Nutrition. 2023 Feb 22:S0022-3166(23)12622-8.
    4. Sonnenburg ED, Sonnenburg JL. The ancestral and industrialized gut microbiota and implications for human health. Nature Reviews Microbiology. 2019 May 17:383–90.
    5. Centers for Disease Control and Prevention (CDC). National Center for Health Statistics (NCHS). NHANES – About the National Health and Nutrition Examination Survey. Hyattsville, MD: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, 2002,
    6. Pei R, DiMarco DM, Putt KK, Martin DA, Gu Q, Chitchumroonchokchai C, White HM, Scarlett CO, Bruno RS, Bolling BW. Low-fat yogurt consumption reduces biomarkers of chronic inflammation and inhibits markers of endotoxin exposure in healthy premenopausal women: a randomised controlled trial. British Journal of Nutrition. 2017 Dec 118(12):1043–51.
    7. Wastyk HC, Fragiadakis GK, Perelman D, Dahan D, Merrill BD, Feiqiao BY, Topf M, Gonzalez CG, Van Treuren W, Han S, Robinson JL. Gut-microbiota-targeted diets modulate human immune status. Cell. 2021 Aug 184(16):4137-4153.e14.
    8. Berding K, Bastiaanssen TFS, Moloney GM, Boscaini S, Strain CR, Anesi A, Long-Smith C, Mattivi F, Stanton C, Clarke G, Dinan T, Cryan JF. Feed your microbes to deal with stress: a psychobiotic diet impacts microbial stability and perceived stress in a healthy adult population. Molecular Psychiatry. 2022 Oct 28:601–10.

    The views and opinions expressed in this publication are those of the contributing authors and editors and do not necessarily represent the views of their employers or IMGC sponsors.”

    Mom’s Helpful Microbes: Yogurt Consumption During Pregnancy Could Lower Risk of Allergies in Offspring

    • A prospective study of over 2,000 mothers and their infants found that yogurt-eating during the last trimester of pregnancy was associated with a lower risk of infantile eczema.
    • As a fermented food, yogurt increases the number of beneficial bacteria in the pregnant mother’s gut microbiome.
    • Researchers hypothesize that the pregnant mother’s gut microbes communicate with the fetal immune system and play a role in the development of allergies. Beneficial bacteria could help prevent allergy-prone phenotypes.

    Pregnant mothers are often told they are eating for two. Although a myth from a caloric perspective (the energetic cost of pregnancy is only 300–400 extra calories per day [1], not double the mom’s caloric needs), there is truth to this old saying. The fetus eats what the mother eats: glucose, amino acids, and fatty acids from the mother’s breakfasts, lunches, and dinners travel from the maternal bloodstream across the placenta and fuel fetal growth and development. 

    But the mom’s food choices may do more than just help grow bones, skin, muscles, and organs. Mounting evidence [2-7] suggests the maternal gut microbiome—the composition of which is strongly influenced by the mother’s dietary choices—helps shape the development of the fetal immune system. Foods that improve the health of the mom’s gut microbiome, like probiotic-rich yogurt, are predicted to direct the immune system of the developing fetus away from inflammatory-prone responses, like allergies [5-7]. It sounds too good to be true, but adding foods like yogurt to the pregnant mother’s diet may be an important tool in allergy prevention.

    A new study [2] tested this intriguing hypothesis in a population of Chinese mothers who were a part of the Tongji Maternal and Child Health Cohort (TMCHC) in Wuhan, China. Just over 2,300 mother-infant dyads were followed from 16 weeks of gestation through three months postnatally, and 2,114 of these dyads were followed for an additional three months. The researchers were specifically interested in the relationship between the mothers’ yogurt consumption during the last trimester of pregnancy and the incidence of eczema, an inflammatory skin condition, in the offspring at 3 months and 6 months of age. Eczema is typically the first sign of allergies in infants who will be prone to allergic reactions and was used by the research team as a proxy for an infant with an allergy-prone phenotype [2]. 

    As predicted, yogurt consumption in pregnancy demonstrated a protective effect. Infants born to mothers who consumed any yogurt during their last three months of pregnancy had a lower risk of having an infant with eczema between three and six months of age compared with mothers who did not consume yogurt [2]. The investigators also identified a dose-dependent effect of yogurt consumption. Every one time per week increase in yogurt consumption was associated with a 2% lower risk of infant eczema and every 10 grams per day increase was associated with a 3% lower risk of eczema in infants three to six months of age, and the lowest relative risk for eczema was associated with infants whose mothers had frequent yogurt consumption (defined in this study as at least three times per week or >50 grams of yogurt per day) [2]. These findings were statistically significant even after controlling for potential cofounders including mother’s age, breastfeeding duration, maternal history of allergies, and probiotic supplementation, and established risk factors for infantile eczema like infant sex, birth season, or mode of delivery [2]. 

    The results of the TMCHC study are in line with previous research on mother-infant dyads from Turkey [8] and Japan [9], both of which identified a protective effect of yogurt consumption during pregnancy on the development of infantile eczema. However, all three of these were prospective studies and did not investigate the mechanisms that linked the maternal gut microbiome with the fetal immune system. As a fermented food, yogurt consumption is understood to improve the composition of the mom’s gut microbiome by increasing the number of healthy gut microbes. Yogurt-eating has been associated with numerous physiological, metabolic, and immunological benefits to the consumer, including a lower risk for type 2 diabetes, cardiovascular disease, chronic inflammation, and dementia. But how do the microbes in the mother’s gut communicate with the developing fetus? 

    During pregnancy, the mother’s gut epithelium (the layer that separates her gut from her bloodstream) becomes more permeable. This physiological change is believed to facilitate greater interactions between the mom’s gut microbiome and her immune system [7]. It is well known that mothers transfer their own immunoglobulin G (IgG) across the maternal-fetal barrier starting as early as 13 weeks of gestation. Researchers hypothesize that these placentally-transferred IgG are microbe-specific. Depending on the type of microbe, the IgG can educate the fetal immune system to have an inflammatory or anti-inflammatory response [5, 7]. Because eating yogurt increases the population of friendly microbes in the mother’s gut, the fetal immune system is predicted to be directed away from an allergy-prone phenotype [5-7]. 

    Another potential mechanism for crosstalk between mother’s gut and fetus is gut-derived short-chain fatty acids (SCFA). SCFA are produced when gut bacteria in the large intestine break down indigestible dietary carbohydrates. SCFA from human milk are already implicated in allergy prevention in nursing infants. Researchers believe that during pregnancy, SCFA travel from the maternal gut to the fetal thymus (a gland that produces lymphocytes called T cells). There, they push those T cells in a non-inflammatory direction by interacting directly with the proteins that surround DNA [5-7]. By keeping certain sections of DNA turned on (or by turning certain sections of DNA off), SCFA can direct the T cells to develop into regulatory cells (aka Treg) or inflammatory cells. The current hypothesis from researchers working in this field suggests the transfer of SCFA from the maternal gut increases the concentration of fetal Treg cells, a process that is supported by animal models [5, 7]. 

    Whether the links between the maternal gut microbiome and the fetal immune system are microbe-specific IgG, SCFA, or both is still being elucidated. But the potential for the maternal diet to influence fetal immunity is an exciting possibility. Dietary interventions during pregnancy often require mothers to give up certain foods. Adding yogurt seems infinitely easier than avoiding raw fish, deli meat, or soft cheeses and would also help pregnant mothers meet their calcium requirements, provide a source of protein, and improve the composition of her own gut microbiome—the potential for allergy prevention in the offspring seems to be an amazing bonus of “eating for two.”


    1. Dufour DL, Sauther ML. Comparative and evolutionary dimensions of the energetics of human pregnancy and lactation. American Journal of Human Biology. 2002 Sep; 14(5): 584-602.
    2. Tan T, Xiao D, Li Q, Zhong C, Hu W, Guo J, Chen X, Zhang H, Lin L, Yang S, Xiong G. Maternal yogurt consumption during pregnancy and infantile eczema: a prospective cohort study. Food & Function. 2023; 14(4): 1929-36.
    3. Vuillermin PJ, Macia L, Nanan R, Tang ML, Collier F, Brix S. The maternal microbiome during pregnancy and allergic disease in the offspring. In: Seminars in Immunopathology 2017 Nov (Vol. 39, pp. 669-75). Springer Berlin Heidelberg.
    4. Macpherson AJ, de Agüero MG, Ganal-Vonarburg SC. How nutrition and the maternal microbiota shape the neonatal immune system. Nature Reviews Immunology. 2017 Aug; 17(8): 508-17.
    5. Kalbermatter C, Fernandez Trigo N, Christensen S, Ganal-Vonarburg SC. Maternal microbiota, early life colonization and breast milk drive immune development in the newborn. Frontiers in Immunology. 2021 May; 12: 683022.
    6. Nyangahu DD, Jaspan HB. Influence of maternal microbiota during pregnancy on infant immunity. Clinical & Experimental Immunology. 2019 Oct; 198(1): 47-56.
    7. Thomson CA, Mccoy KD. The maternal microbiome. The Scientist. 2021 Aug 1;35(4): 32-8. 
    8. Celik V, Beken B, Yazicioglu M, Ozdemir PG, Sut N. Do traditional fermented foods protect against infantile atopic dermatitis. Pediatric Allergy and Immunology. 2019 Aug; 30(5): 540-6.
    9. Miyake Y, Tanaka K, Okubo H, Sasaki S, Arakawa M. Maternal consumption of dairy products, calcium, and vitamin D during pregnancy and infantile allergic disorders. Annals of Allergy, Asthma & Immunology. 2014 Jul 1; 113(1): 82-7.

    The views and opinions expressed in this publication are those of the contributing authors and editors and do not necessarily represent the views of their employers or IMGC sponsors.”

    The Tall and the Short of It: Can Milk Drinking Explain Increases in Stature Among Prehistoric Agriculturalists?

    • The transition from hunting and gathering to agriculture had profound changes on the human diet that are believed to have negatively impacted human health, including growth rates.
    • A new study on over 3,500 skeletal remains across Europe, Asia, and Northern Africa spanning the last 34,000 years suggests that declines in stature and body size preceded the adoption of agriculture in many regions, and in some places stature and body size were stable over time.
    • Increases in body size and stature in Central and Northern Europe that began approximately 8,000 years ago were argued to be the result of milk drinking, facilitated by selection on lactase persistence genes among these populations.
    • The ability to drink milk without digestive issues may have buffered the nutritional stresses associated with a farming mode of subsistence.

    You may have heard the claim that if you double your child’s height at age two, you can determine how tall they will be as an adult. Before you go and check the pencil marks on the doorframe at your childhood home to confirm, take note: this method for estimating height fails to consider the fact that adult stature is a complex trait that results from the interaction of genes and environment [1]. Not every three-foot-tall two-year old grows up to be a six-foot-tall adult—a taller than average toddler could have stunted growth due to chronic infections or inadequate nutrition during childhood and adolescence. 

    Because of the influence of the environment on an individual’s growth, the World Health Organization uses height throughout childhood and adolescence as a measure of health in at risk populations. For the same reasons, archaeologists also relate height to health in prehistoric populations. A long-standing hypothesis in anthropology argues agricultural populations will be shorter than their hunter-gatherer ancestors because of nutritional deficiencies that accompanied the shift from foraging to farming (e.g., decrease in dietary breadth, increase in calories coming from carbohydrates compared with protein). A new study [2] challenges this assumption and instead proposes that agricultural populations that consumed animal milk and had the genetic variant allowing for lactase digestion (aka lactase persistence, or LP) would be taller than their non-dairying, non-LP contemporaries. Called the Lactase Growth Hypothesis, it predicts that the shift to milk consumption, facilitated by selection for LP, would have “turbo-charged” growth rates by making up for any nutritional or immunological challenges that accompanied an agricultural mode of subsistence [2]. The tallest modern-day Europeans are those that get more protein from milk and less protein from wheat [3]. Could the same be true among Neolithic farmers?

    To test the Lactase Growth Hypothesis, researchers examined changes in stature and body mass from over 3,500 skeletal remains representing 366 archaeological sites across Europe, Western Asia, Northern Africa (Nile Valley), South Asia, and China. The dataset ranged from 34,000 years ago to present, including Upper Paleolithic (Late Pleistocene) foragers, populations in transition from foraging to farming (early Neolithic), and populations that relied solely on agriculture (including herding) for subsistence. Estimates for stature and body mass of each skeleton were calculated using established regression equations that derive overall height from the length of the femur or tibia and overall body size from the diameter of the femoral head or tibial plateau [2]. 

    The study authors identified a long-term decrease in average height and body mass that preceded the transition to agriculture, starting in the Upper Paleolithic (20,000–15,000 years ago) in most of the regions examined [2]. Moreover, not every region demonstrated the same response to the transition to agriculture in body size. In areas known to be associated with independent adoption of agriculture (rather than cultural transmission), stature and body size were relatively stable. On the other hand, regions where non-native crops were introduced, like Northern Europe, experienced declines in stature. Together, these findings suggest that the pace (gradual vs. rapid) and trajectory (introduced vs. independent development) of the transition to agriculture must be considered when discussing the effect of agriculture on human health. The classic dichotomy of tall foragers and short farmers was not supported by this dataset. 

    Importantly, the study also reported that following declines in stature, some farming populations demonstrated increases in body size over time [2]. Northern Europe (from 7,000–4,000 years ago) and Central Europe (from 8,000–5,000 years ago) experienced the most significant increases in height and body mass compared with other regions during these same time periods. The timing and geography of these stature increases correspond with archeological evidence for dairy farming and genetic evidence for selection on LP, supporting the predictions of the Lactase Growth Hypothesis [2]. 

    The study authors believe that environmental stressors that could impact growth, including crop failure or lack of diversity of crops (both of which could have been common in more northern latitudes), were buffered by milk drinking in these European LP populations [2]. Individuals that lack LP genes can consume milk, but the side effects (particularly diarrhea) would be especially problematic during times of nutritional stress. Dairy foods with reduced lactose content, like yogurt and cheese, would allow consumers to avoid digestive issues but may not have the same impact on skeletal growth because fermentation destroys a key ingredient: insulin-like growth factor I (IGF-I). IGF-I is a growth hormone, present in mammal milk to support infant growth [4]. Cow milk consumption by humans is associated with increases in circulating IGF-I and increased IGF-I is known to promote skeletal growth [2, 4]. As a result, animal milk consumption after weaning (and as a weaning food) would be associated with continued exposure to IGF-I, which is predicted to mediate skeletal growth throughout childhood and adolescence [2, 4]. 

    Milk drinking and LP selection coincide with the observed increases in body size, but are they a coincidence or the cause? The study design doesn’t allow the authors to do more than speculate; they did not collect ancient DNA (aDNA) from the skeletons they measured so they cannot say with certainty that the changes over time in Central and Northern Europe were exclusively in individuals with LP gene variants. Additionally, the authors concede that there are other, non-dietary explanations for the increases in stature observed among the Northern and Central Europeans, including gene flow with other populations (new gene variants introduced by mating with other populations), genetic drift (some genes randomly increase in frequency because of small population size), or genetic changes in energetic allocation to growth, immune function, and reproduction [2]. These factors could be operating alongside selection for LP in explaining stature increases or instead of selection for LP. 

    An important next step to untangle these potential cofounding factors would be to test the Lactase Growth Hypothesis in other populations that depended heavily on milk, rather than cheese or yogurt, for nutrition over the last 10,000 years and underwent rapid selection for LP gene variants. The study authors are particularly interested in understanding if LP and milk drinking explains variation in body size among modern-day East African populations, including the Maasai herders of East Africa. Unfortunately, the data required to search for trends in stature over the last 30,000 years are currently lacking for this population. Testing the Lactase Growth Hypothesis among non-European populations is critical to understanding how milk drinking and the evolution of LP shaped body size variation in modern humans.


    1. Visscher PM. Sizing up human height variation. Nature Genetics. 2008;40: 489-490.
    2. Stock JT, Pomeroy E, Ruff CB, Brown M, Gasperetti MA, Li FJ, Maher L, Malone C, Mushrif-Tripathy V, Parkinson E, Rivera M, Siew YY, Stefanovic S, Stoddart S, Zarina G, Wells JCK. Long-term trends in human body size track regional variation in subsistence transitions and growth acceleration linked to dairying. Proceedings of the National Academy of Sciences. 2023 Jan 24;120(4): e2209482119.
    3. Grasgruber P, Cacek J, Kalina T, Sebera M. The role of nutrition and genetics as key determinants of the positive height trend. Economics and Human Biology. 2014;15: 81-100.
    4. Wiley AS. The evolution of lactase persistence: milk consumption, insulin-like growth factor I, and human life-history parameters. The Quarterly Review of Biology. 2018 Dec 1; 93(4): 319-345.

    The views and opinions expressed in this publication are those of the contributing authors and editors and do not necessarily represent the views of their employers or IMGC sponsors.”

    Cell Types and Metabolites in Human Milk Change Over Lactation

    • The ratio of two populations of epithelial cells in human milk changes over the course of lactation, with one population predominant earlier in lactation and the second becoming more prevalent later.
    • These changes in two cell types may reflect structural changes occurring in the breast over months of lactation.
    • Metabolite levels in human milk also continuously vary throughout lactation.

    Human milk is a highly nourishing food for infants and babies—not only does it fulfill their nutritional needs, but it also provides extensive health benefits. Although human milk is known to contain a diverse population of cells and metabolites (1), scientists are just beginning to characterize these components in the hope of better understanding their health-promoting properties as well as the relationship between maternal and infant wellness. 

    Advanced molecular techniques allow researchers to identify milk’s cellular components in unprecedented detail. A few years ago, researchers first used an approach called single cell RNA sequencing (scRNAseq) to investigate gene expression in individual cells from milk of two post-partum women, identifying a surprising diversity of cell types (2). Since then, a handful of studies have also investigated the cellular fraction of human milk using this technique. “All of the studies have nicely validated each other,” said Britt Goods, an assistant professor of engineering at Dartmouth College in Hanover, New Hampshire. 

    In one such study published last year, Goods and her colleagues used scRNAseq to track how the cellular components of human milk changed at different stages of lactation (3). The researchers analyzed milk samples from 15 women taken over approximately 21 months at multiple timepoints during lactation. The samples included the early milk that comes in during the first 5 days postpartum, transitional milk that is present 6-14 days after delivery during a period of rapid change in breast tissue, mature milk that comes in after about 2 weeks, and late-stage milk from mothers who are weaning their babies to solid foods. 

    The researchers identified two different populations of epithelial cells called lactocytes whose proportions shift over the duration of breastfeeding, suggesting structural changes occurring in the breast during that time (3). One of these lactocyte populations, called LC1, increased as a function of time postpartum while the other, called LC2, decreased. Gene expression profiles from the cells in each group suggest that cells in the LC1 cluster provide structural support for the breast during later stages of lactation—for example, playing a role in the formation of tight junctions between cells—whereas LC2 cells produce milk components. Goods cautions, however, that she and her colleagues have not yet validated this hypothesis and that more milk samples would be needed to draw firm conclusions. Day care attendance and the use of hormonal birth control were also associated with the predominance of LC1 levels.

    The researchers also identified a significant presence of immune cells called macrophages, which play a role in responding to tissue injury. “We think immune cells are taking on these anti-inflammatory phenotypes to support the breast as it’s responsible for making this tremendous amount of milk,” Goods said. There’s no evidence currently that these cells transfer to the infant, but it is possible that they do, and that they secrete molecules there that support gut health, she added.

    Overall, the researchers found a highly diverse population of epithelial cells in human milk, and found that this diversity increased in later stages of lactation. “This could suggest some specialization of these cells happening over time, where these cells might be making or secreting substances or dividing the labor a bit differently in earlier compared to later time points,” Goods said. She and her colleagues are now following up on some of these findings, particularly functional differences in the LC1 and LC2 populations. “The big question now is what does this all mean from a biology standpoint,” Goods said.

    Meanwhile, other studies are exploring the cellular content of human milk by analyzing its metabolome, that is, by characterizing the sugars, lipids, amino acids, and other molecules it contains. One study, published in 2022, analyzed 101 milk samples collected from 59 mothers over 25 months. The researchers observed a dynamic pattern over the course of lactation, with some metabolites present at higher levels earlier in lactation and others at higher levels later on [4]. “Overall, we find that the metabolome of [human milk] continuously varies throughout lactation and some changes are distinctive for milk excreted after 1 to 2 months of lactation,” the authors write. 

    A more recent study, published in 2023, took a slightly different approach (5). Here, the researchers analyzed 15 human milk samples collected at different postnatal times in the first year of lactation. However, unlike in the previous study, where researchers extracted metabolites from the entire milk sample, in this study they first isolated the cells and then extracted the metabolites only from the milk’s cell fraction. 

    The team noted that cell counts in the samples decreased along lactation (5), an observation that confirmed previous work (6). Because the overall number of cells was low, it was difficult to conduct a full metabolic analysis, said Isabel Ten-Doménech, a postdoctoral researcher studying neonatal health at the Health Research Institute Hospital La Fe in Valencia, Spain, and the first author of that work. Overall, however, the team found a decrease in the number of metabolites detected over time. Out of the 53 total metabolites identified, the study found two that became more prevalent during the sampling year [5]. “We hypothesize that this is because these two metabolites are related with cell death,” said Ten-Doménech. She also noted that the study’s small sample size made its conclusions preliminary. 

    Just as nursing infants continually change, the cells and metabolites of their mother’s milk also change. Better understanding of these changes in milk will yield tools for promoting healthy lactation and improving infant nutrition.


    1. Witkowska-Zimny M, Kaminska-El-Hassan E. Cells of human breast milk. Cell Mol Biol Lett. 2017;22(1):1-11.
    2. Martin Carli JF, Trahan GD, Jones KL, Hirsch, N, Rolloff, KP, Dunn, EZ, Friedman, JE. Barbour, LA. Hernandez, TL, MacLean, PS. Monks, J. Single cell RNA sequencing of human milk-derived cells reveals sub-populations of mammary epithelial cells with molecular signatures of progenitor and mature states: a novel, non-invasive framework for investigating human lactation physiology. J Mammary Gland Biol Neoplasia. 2020;25(4):367-387.
    3. Nyquist SK, Gao P, Haining TKJ, Retchin MR, Golan Y, Drake RS, Kolb K, Mead BE, Ahituv N, Martinez ME, Shalek AK, Berger B, Goods BA. Cellular and transcriptional diversity over the course of human lactation. Proc Natl Acad Sci U S A. 2022; 119(15):e2121720119.
    4. Poulsen KO, Meng F, Lanfranchi E, Young JF, Stanton C, Ryan CA, Kelly AL, Sundekilde UK. Dynamic changes in the human milk metabolome over 25 weeks of lactation. Front Nutr. 2022;9:917659.
    5. Ten-Doménech I, Cascant-Vilaplana MM, Navarro-Esteve V, Felderer B, Moreno-Giménez A, Rienda I, Gormaz M, Moreno-Torres M, Pérez-Guaita D, Quintás G, Kuligowski J. Metabolomic diversity of human milk cells over the course of lactation-a preliminary study. Nutrients. 2023 Feb 22;15:1100.
    6. Hassiotou F. Hepworth AR, Metzger P, Tat Lai C, Trengove N, Hartmann PE, Filgueira L. Maternal and Infant Infections Stimulate a Rapid Leukocyte Response in Breastmilk. Clin Transl Immunol. 2013 Apr 12; 2(4), e3.

    The views and opinions expressed in this publication are those of the contributing authors and editors and do not necessarily represent the views of their employers or IMGC sponsors.”

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